As important instruments for the treatment of stenosis, stents have found increasingly extensive use in the field of cardiovascular diseases. Metal stents, that are widely used in the contemporary clinical practices, will permanently exist in the body after the fulfillment of their therapeutic mission, and are thus associated with a number of drawbacks such as, for example, deteriorating coronary MRI or CT images, hindering surgical revascularization, impeding collateral circulation and inhibiting positive vessel remodeling. Biodegradable stents have aroused widespread attention as a possible alternative solution.
A biodegradable stent is fabricated from a degradable polymeric or metallic material, and after its deployment to a treatment site, the biodegradable stent can support a vessel for a limited period of time to allow revascularization thereof. After the treatment regimen, the biodegradable stent will degrade in the body into absorbable or metabolizable organic substances and eventually disappear.
Biodegradable polymeric materials commonly used for fabricating such stents include polylactic acid (PLA), polyglycolic acid (PGA) and polycaprolactone (PCL), and biodegradable metallic materials commonly used for fabricating such stents include magnesium alloys and iron-based alloys. Magnesium and iron are essential trace elements in living organisms, which have good biocompatibility, unique degradation and absorption properties, excellent mechanical properties and molding properties.
However, it is found in practical use that, polymer stents generally have poorer mechanical properties and larger sizes compared with metal stents, and are more likely to induce vascular lumen loss, local inflammation and intimal hyperplasia. Moreover, polymer stents are not radiopaque. On the other hand, magnesium alloys stents have been found degrading too fast, which results in a too fast decay of the mechanical strength of magnesium alloys stents in the body and the effect of treatment is affected. Although the mechanical property of iron-based alloy stent meets the requirement, the corrosion rate of iron-based alloy stent is difficult to control, and the corrosion degradation mechanisms in the simulated body fluid and human environment are unknown, thus limiting the application of iron-based alloy stent as cardiovascular stent.
In order to overcome the problem of too fast degradation of magnesium alloys stents, U.S. Patent Publication No. US2009240323A1 describes a method of coating a biodegradable polymer on the surface of magnesium stent which can realize controllable degradation of magnesium stent. However, the polymer coating is required to be dense enough to prevent the body fluid from permeating through the biodegradable polymer coating into the magnesium stent to cause the degradation of the magnesium stent.
In order to solve the problem of weak mechanical properties of biodegradable polymer stents, U.S. Patent Publication No. US20110015726 describes a method of improving the mechanical properties of biodegradable stents which uses ceramic materials serving as reinforcing materials to add in biodegradable materials. However, the degradation mechanism of ceramic material in blood is not clear, and toughness of the material will be significantly decreased. Another U.S. Patent Publication No. US2009248147A1 describes a method of improving the mechanical properties of biodegradable copolymer stents which uses homopolymer and inorganic salt as nucleating agents to add in biodegradable copolymer materials. But the crystallization rate of the copolymer itself is lower than that of the homopolymer, and therefore the mechanical strength of the stent is not significantly improved by the addition of nucleating agent.
In order to solve the radiopacity problem, U.S. patent publication No. US2009149940A1 disclosed a method of coating a radiopaque layer with radiopaque particles on the surface of a stent to achieve the overall radiopacity of the stent. However, the mechanical strength of the radiopaque layer is relatively low, and the radiopaque efficiency is relatively poor. The coating may also have the defect of bonding too tight with the main body of the stent.
In addition, Chinese patent publication No. CN102532835A disclosed a method of casting molding a stent using a composite prepared by solution blending polylactic acid and magnesium. This method uses magnesium as an inorganic medium to improve the mechanical properties of PLA stent, and to neutralize the acidic substances produced from the degradation of PLA. However, due to the strength of magnesium is not much higher than PLA, and both have poor interface compatibilities, therefore, the improvement of the mechanical properties is limited. Further, the method fails to effectively utilize the crystallization properties of PLA, resulting in a relatively low mechanical strength of the stent.
Thus, there is still a need for a biodegradable stent having a better performance in the prior art.